During combustion in a fluidized bed, the combustion temperature must be maintained below the level at which slag formation arises and slag lumps are formed, which disturb the fluidization and render impossible the discharge of residual products. The upper temperature limit during combustion of coal in a fluidized bed varies with the coal quality and is usually between about 850.degree. and 950.degree. C. This means that the propellent gas for the turbines included in a PFBC power plant has a temperature which is approximately equal to the temperature in the fluidized bed. As the turbine power increases steeply with increasing temperature of the propellent gas, a considerably higher propellent gas temperature, up to about 1200.degree. C., is desirable in order to achieve a high power output in the gas turbine section of the plant.
From EP No. 0 144 172 it is known to attain a temperature of 1350.degree. C. in gases leaving a combustion chamber burning coal in a fluidized bed. The combustion chamber is intended to generate gases for a process industry. The bed vessel is divided into two sections. One section is driven as a gas generator, the gas of which being burnt in a freeboard common to both sections. Dust in the gas seems to have no importance since no cleaning plant is shown.